It is well-known to employ 2,4-dialkyl imidazole as a curing catalyst in phenol-curable epoxy compositions. However, the presence of a curing catalyst in a curable composition in storage tends to induce some degree of curing thereby reducing storage time. Furthermore, in use, heating to the curing temperature results in significant variation in cure rate from place to place thereby leading to inhomogeneities in the cured object.
It is known in the art to employ latent catalysts in order to bring the above stated problems under control. A latent catalyst is one that does not catalyze curing of the epoxy composition until some threshold condition, typically a threshold temperature, is reached. However, the requirements for a latent catalyst to be of practical use are stringent. A latent catalyst should not be activated until the desired curing temperature is reached, typically in the range of 130 to 180° C. In addition, the catalyst, to be useful in high value applications in electronics, needs to be effective in curing reactions that do not produce gaseous by-products which can create bubbles in the cured layer. The catalyst needs to be soluble in the solvents, such as methyl-ethyl ketone (MEK) that are employed in common epoxy compositions.
Shigemitsu et al., JP1999343476(A), discloses a curable composition comprising a 2,4 alkyl-substituted imidazole, benzoic acid, an epoxy, and amine curing agent. The benzoic acid is employed in considerable excess over the imidazole to reduce the water sensitivity of the amine curing agent.
Amano et al., US 2009/0030158, discloses a curing catalyst for epoxy compositions comprising a 1,3-alkyl-substituted monocarboxylic acid.